Precision Servo-Hydraulic Control Using NI LabVIEW FPGA and PXI Hardware

Author(s):
Dr. Martin Saxon - Product Technology Partners Ltd.

Industry:
Manufacturing, Consumer Goods

Products:
PXI-7344, PXI-6515, PXI-7833R, PXI-6123, PXI-6220, FPGA Module, LabVIEW, Real-Time Module

The Challenge:
Designing a low-cost data acquisition and servo-hydraulic control system for a pharmaceutical tablet manufacturing machine.

The Solution:
Using NI PXI hardware and LabVIEW graphical programming software to prototype a servo-hydraulic control system in less than three days, improving system performance and significantly reducing hardware and software complexity and cost.

The manufacturing of pharmaceutical tablets is carried out using machines that appear, at first sight, to be very simple. First, a die is filled with the correct amount of powder. Next, two opposing punches compress the powder to form the tablet. Finally, the tablet is ejected from the top of the die. In a typical manufacturing machine, rotating mechanical cams control the movement of the punches.

The apparent simplicity of the manufacturing process hides extensive development that pharmaceutical companies undertake for each different formulation to ensure that the tablet is properly formed, the active ingredients are not denatured, the tablet does not disintegrate between formation and ingestion, and that it dissolves or breaks up at the correct time when swallowed. To fulfil all of these requirements, the principal characteristics of the manufacturing process must be established: the speed at which the tablet should be formed, the cam profiles used to drive the punches, and the minimum distance between the punch faces (which of course determines the thickness of the tablet). To do all of this, pharmaceutical companies use a compaction simulator.

The Compaction Simulator

In the compaction simulator, high-performance servo-hydraulic rams instead of mechanical cams drive the punches. With this arrangement, engineers can quickly evaluate different punch movement profiles, speeds, and approach distances to easily adjust all of these parameters using the simulator’s operating software. To fully automate the simulator and, therefore, generate a number of tablets over a range of conditions, pneumatic and stepper motor-driven subsystems control the operation of powder-filled hoppers, a tablet removal arm, a tablet storage magazine, and other sundry items. The simulator must be fully instrumented so position, force, and temperature profiles may be recorded for the formation of each tablet.

The servo-hydraulic performance of the simulator is critical to the successful operation of the machine. Tablet manufacturing machines operate at a high speed. For example, a typical upper punch profile might require the punch to descend 15 mm in 15 ms then immediately reverse back to the starting position at the same speed. This profile must be well-controlled even though the punch load increases rapidly when the punch hits the powder. For this reason, engineers use very low friction hydraulic rams and high-performance servo valves (~1 ms time constant).

The Machine Control System

We at Product Technology Partners (PTP) developed the original control, data acquisition, and analysis software for the compaction simulator. The control and data acquisition system for the machine is complex, consisting of the following main items:

•  A special purpose, high-performance, two-axis servo-hydraulic controller for the punches

•  A special purpose, three-axis stepper motor controller for control of the powder hopper, tablet collection arm, tablet magazine, and various pneumatic actuators

•  An industrial PC providing supervisory control, coordination, data acquisition, multiple control functions, and the human machine interface (HMI)

Previously, to achieve the required machine control, engineers heavily interlinked the control system in a rather complex arrangement. It therefore suffered from the following deficiencies in both performance and maintainability:

• The lack of flexibility in the servo-hydraulic controller limited performance

• The data acquisition subsystem and the controllers had to be separately calibrated

• The controller structure required extensive wiring and complex application software that is costly to develop and maintain

• The PC was required to undertake real-time control and synchronization functions for which it is not well-suited

• The control system hardware is costly

A New Approach to Control

Having established the deficiencies of the existing control system, we were asked if we could simplify the system using different hardware, and we undertook a study to establish an alternative controller design.

Starting from a blank sheet, it quickly became apparent that we could effectively address these identified deficiencies with a system based on an NI PXI controller running LabVIEW Real-Time and combined with a host PC to provide the HMI. The new system offered the following benefits:

•  Precise control of the punches using an NI R Series PXI module programmed with LabVIEW FPGA technology for advanced, nonlinear control algorithms

•  Hardware reuse by using the same R Series module for data acquisition control and avoiding the requirement for separate calibration

•  Containment of the complete controller within a single LabVIEW Real-Time system to dramatically reduce both wiring and application software complexity

•  All real-time tasks undertaken within a real-time environment

•  A significant cost saving (approximately £20,000 per unit) with the new hardware configuration versus the old

The I/O configuration of our PXI controller is as follows:

•  NI PXI-7833R R Series module for servo-hydraulic control and fast data acquisition

•  NI PXI-6123 S Series data acquisition module for additional fast data acquisition

•  NI PXI-6220 M Series data acquisition module for auxiliary data acquisition

•  NI PXI-7334 stepper motion controller for hopper, tablet removal, and tablet magazine control

•  Two NI PXI-6515 digital I/O modules for pneumatic and safety systems control and monitoring

•  NI PXI-8420/8 RS232 S Series interface for communication with optional machine subsystems (for example, die heater controller)

The change in controller required redevelopment of the machine operating software. However, we calculated that the cost of redevelopment would be recovered from the reduction in hardware cost with the sale of only two machines.

Servo-Hydraulic Control Performance

Prior to implementation of the revised controller, the machine manufacturer required assurance that the R Series module, programmed with LabVIEW FPGA, was capable of at least matching the performance of the existing, special purpose controller.

To provide this assurance, we constructed a prototype to control the punches of an existing machine using a PXI-7833R module. We developed a simple controller in LabVIEW based on PID control. We combined it with static compensation for the nonlinear servo-valve characteristic and gain scheduling based on the pressure difference across the valve. It took us only about 2.5 man-days to develop the prototype and the trial software. This fact reflects the simplicity of the LabVIEW FPGA approach to application development. However, it must be noted that we needed to take significant care with the algorithm development because LabVIEW FPGA supports only integer and fixed-point arithmetic.

We then conducted comparison measurements using both the experimental controller prototype and the existing controller to perform V-profiles. We executed profiles at several different speeds to form tablets using different maximum punch loads.

From a comparison of the error span results, we showed that, with only two days of software development effort, the performance of the LabVIEW FPGA test controller prototype exceeded that of the existing controller when under load. Under no-load conditions, the demonstrated performance was not quite as good, but we are confident that this could be improved through the application of a more sophisticated control algorithm.

We concluded that it is very easy to configure the LabVIEW FPGA controller to match the performance of the special purpose controller. With advanced control system design and further software development, we are confident that we could substantially enhance performance.

A complex R&D machine such as a compaction simulator places many demands on its control system, including precision servo-hydraulic control; high-speed data acquisition; and control, sequencing, and monitoring of the auxiliary subsystems. In comparison to a conventional control hardware design, which combines special purpose controllers and a PC in a real-time role, we demonstrated that an integrated controller embracing PXI, LabVIEW Real-Time, and LabVIEW FPGA technologies can have significant benefits, including:

• Reduction of hardware and software complexity

• Significant reduction of hardware cost

• Significant reduction of software development time

• The ability to implement advanced control designs, incorporating linear and nonlinear elements, with FPGA technology to provide precision servo-hydraulic control over a wide range of operating conditions

We would like to thank National Instruments for the loan of equipment and its invaluable assistance during this design study.

Author Information:
For more information on this Case Study, contact:
Dr. Martin Saxon
Product Technology Partners Ltd.
Barrington Road
Orwell Royston SG8 5QP
Tel: +44 (0) 1223 208791
martin@ptpart.co.uk

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